Magnetic random access memory (MRAM) devices are solid state, non-volatile memory devices which make use of the giant magnetoresistive effect. A conventional MRAM device includes a column of first electrical wires, referred to as word lines, and a row of second electrical wires, referred to as bit lines. An array of magnetic memory cells, located at the junctions of the word lines and bit lines, is used to record data signals.
A magnetic memory cell includes a hard magnetic layer, a soft magnetic layer, and a non-magnetic layer sandwiched between the hard magnetic layer and the soft magnetic layer. The hard magnetic layer has a magnetization vector or orientation fixed in one direction. The orientation of this magnetization vector does not change under a magnetic field or electron spin-torque applied thereon. The soft magnetic layer has an alterable magnetization vector or orientation under a magnetic field applied thereon, that either points to the same direction, hereinafter “parallel alignment”, or to the opposite direction, hereinafter “antiparallel alignment”, of the magnetization vector or orientation of the hard magnetic layer. Since the resistances of the magnetic memory cell in the “parallel alignment” status and the “antiparallel alignment” status are different, the two types of alignment status can be used to record the two logical states—the “0”s or “1”s of a data bit.
In a writing operation for one illustrative embodiment, an electric current passes through the word line and the bit line adjacent to the memory cell. When the electric current reaches a certain threshold, a magnetic field generated by the electric current will switch the orientation of the magnetization vector of the soft magnetic layer. As a result, the magnetization vector of the hard magnetic layer and the soft magnetic layer will be changed from one type of alignment, e.g. “parallel alignment”, to the other type of alignment, e.g. “antiparallel alignment”, so that a data signal in form of one data bit can be recorded in the memory cell.
In MRAM structure design, lower writing power dissipation and a higher cell density are desired. Unfortunately, a reduction of cell size, i.e. an increase in cell density, leads to a reduction in the available energy (KuV) to store the bit data. Further, the error rate increases as the cell size scales down. In order to reduce the error rate, high anisotropy energy is required to overcome thermal fluctuations. Hard magnetic material has higher anisotropy energy compared with soft magnetic material, but in that case a higher writing current is required. The higher anisotropy energy results in higher writing current density, which unfortunately has the disadvantage of electro-migration.
In order to reduce the writing current for a high coercitivity MRAM, thermally assisted MRAMs are disclosed. Un-pinned ferromagnetic materials, in which the coercitivity decreases sharply as temperature increases, are used for the recording layer in the MRAMs.
Another type of MRAM is spin-transfer torque memory (STRAM). STRAM utilizes electron spin torque to switch the free layer by passing a spin polarized current thorough the STRAM. STRAM has a higher efficiency as the memory cells scale down, but still suffers from the same issues as other MRAM cells as STRAM scales down. STRAM can also utilize thermal assist concept to reduce the switching current and maintain data retention time.
However, thermally assisted MRAM suffer from low heating efficiency. In addition, due to Joule heating, heat gradually builds in the memory array structure which increases the temperature of the memory device during operation.